Preheat the oven to 180oC and line a pie dish with parchment paper.
In a large frying pan fry the chicken in some oil and season with salt and pepper until crispy.
Remove and place into the oven for 30mins to cook through.
In the same pan sauté the garlic, thyme and leeks until softened.
Add the butter and the flour to the pan and cook out for 1-2 mins. Then add the milk and mustard.
Once the chicken is cooked chop it up and add it to the pan. Season the mixture and place into the pie dish. Lay the puff pastry on top. Score it with a knife and brush with egg wash.
